PHP stands among the prime languages employed in crafting sophisticated web applications. It is an interpreted language, meaning that PHP applications are not pre-compiled. Instead, they are translated into machine code at runtime, on-the-fly. This dynamic nature of PHP makes it a powerful tool for web development but also creates a veil of mystery around the actual conversion process of PHP code into a functional application. This comprehensive guide seeks to pull back that veil and elucidate how a PHP interpreter processes and executes the PHP code.
Understanding Compilation and Interpretation
Computer languages can be broadly categorized into two groups based on how they convert their written code into executable applications – Compiled languages and Interpreted languages.
Compiled Languages: Efficiency at the Cost of Development Time
Compiled languages like C and C++ operate by translating their entire code into machine language in a single conversion process known as Compilation. The translated executable can be run multiple times without needing recompilation, making these languages highly efficient for application execution.
However, this efficiency comes with certain drawbacks. Code changes in compiled languages require an additional step of recompilation before they can be tested, resulting in a slower development process.
Interpreted Languages: Flexibility over Performance
On the other side of the spectrum, we have Interpreted languages like PHP, Python, and Ruby. These languages employ a different strategy for code conversion. Instead of pre-compilation, Interpreted languages use a tool called an Interpreter to translate their code into machine language on-the-fly during runtime.
While this approach allows for easier software development and code flexibility, it also means lower performance and extended application execution time due to the need for constant interpretation.
Delving Into the Zend Engine
The Zend Engine, the heart and soul of the PHP language, is a remarkable piece of technology acting as a source code to bytecode compiler and a virtual machine that executes the produced code. Bundled directly with PHP, installing PHP implies installing the Zend Engine. The entire PHP code processing journey, beginning when your HTTP server sends the requested PHP script execution to the engine, to the moment the HTML code is generated and delivered back to the server, is the Zend Engine’s responsibility.
Let’s simplify this complex process and highlight the four significant stages the PHP interpreter, specifically, the Zend Engine, carries out:
- Lexical analysis (lexing);
- Syntax analysis (parsing);
- Compilation;
- Execution.
To optimize performance, PHP introduces the OPcache mechanism, which essentially allows you to skip all the stages until the final step – launching/execution of the application on the virtual machine. PHP 8 introduces a new feature, the Just-In-Time (JIT) compiler, which makes it possible to compile PHP code that can run machine code directly, bypassing the interpretation process or execution by a virtual machine.
In addition to these, there have been attempts to transpile PHP code, for instance, into C++. Facebook programmers once created HipHop for PHP, which utilized this method. However, as its development ceased, it was succeeded by the HipHop Virtual Machine (HHVM), which employs the Just-In-Time (JIT) compilation.
Now, let’s get a basic understanding of each step of the interpretation process.
The Basics of Each Interpretation Step
- Lexical Analysis (Lexing): This is the first step, where PHP code is broken down into tokens. These tokens are a group of characters that have a collective meaning in PHP syntax;
- Syntax Analysis (Parsing): After the code is broken down into tokens, the parser checks the tokens to ensure they follow PHP’s grammatical rules. If there’s an error, the parser will stop and throw a parsing error;
- Compilation: The correct tokens are then converted into opcodes, which are understandable by the machine;
- Execution: Zend Engine then executes these opcodes one by one. If there’s an error during execution, a fatal error will be thrown.
A Dive into Lexical Analysis in PHP
Lexical analysis, often referred to as ‘lexing’ or ‘tokenizing’, is the step that transforms raw PHP code into a sequence of symbols or ‘tokens’. Each token represents a string of characters that holds a specific meaning within the PHP syntax. This conversion process aids the interpreter in further code processing.
The PHP language employs the re2c lexer generator along with the definition file zend_language_scanner.1. Essentially, it utilizes regular expressions to identify distinct code elements within the syntax such as “if”, “switch”, “function”, and others.
To offer a simplified representation of how tokens are generated, consider the following PHP code:
function lexer($bytes, ...) {
switch ($bytes) {
case substr($bytes, 0, 2) == "if":
return TOKEN_IF;
}
}This is not precisely how the lexer operates, but it provides a general idea regarding the analytical process. Let’s consider a piece of PHP code, and view the corresponding tokens.
For example, the PHP code:
$my_variable = 1;Will be tokenized as:
T_OPEN_TAG ('<?php')
T_VARIABLE ('$my_variable')
T_WHITESPACE (' ')
=
T_WHITESPACE (' ')
T_LNUMBER ('1')
;Note that not all elements are converted into named tokens. Some, such as =, ;, :, ?, are treated as tokens in and of themselves.
Impressively, the lexer doesn’t merely tokenize the code; it also stores values related to the tokens and the specific line of their occurrence. This is instrumental in generating an application’s stack trace, among other things.
Diving into Syntax Analysis
Post lexing, the tokens are transformed into a more structured and organized data architecture through Syntax Analysis, aka ‘parsing’. This process utilizes an external tool, GNU Bison, based on a BNF file incorporating the language’s grammar. This tool transforms the context-free grammar into a consequential one.
Parsing employs the LALR(1) method, reading the input from left to right, previewing one token forward and generating a right-deviated output. In this process, tokens are matched against the BNF file-defined grammar rules, and the correctness of syntax constructs is validated.
The resultant of this process is the generation of an abstract syntax tree (AST), which is a graphical representation of the source code to be utilized in the compilation phase.
Here’s a simple illustration of how to visualize the syntax tree structure using the php-ast extension:
$php_code = <<< 'code'
<?php
$my_variable = 1;
code;
print_r(ast\parse_code($php_code, 30));
The result will bear a structure similar to:
ast\Node Object (
[kind] => 132
[flags] => 0
[lineno] => 1
[children] => Array (
[0] => ast\Node Object (
[kind] => 517
[flags] => 0
[lineno] => 2
[children] => Array (
[var] => ast\Node Object (
[kind] => 256
[flags] => 0
[lineno] => 2
[children] => Array (
[name] => my_variable
)
)
[expr] => 1
)
)
)
)Although the structure might appear complex, it is highly valuable for performing static code analysis using tools like Phan. After the code has been analyzed and AST has been generated, the code is ready for the next phase – compilation.
Conclusion
In a nutshell, the PHP interpreter, spearheaded by the Zend Engine, serves as a linchpin in developing efficient and dynamic PHP applications. Understanding how it transforms your PHP scripts into executable machine code offers invaluable insights. This knowledge aids in not only writing more efficient code but also in leveraging new tools like JIT and OPcache. Digging deeper into this crucial component of PHP indeed opens up a fascinating universe of possibilities and fine-tuning opportunities, making it a must-know aspect for every aspiring PHP expert. If you’re aiming to enhance your programming expertise, consider mastering an Integrated Development Environment (IDE) for PHP.